The present invention generally relates to vibration absorbers. More specifically, this invention relates to a vibration absorber having a characteristic resonant frequency tuned to absorb vibrational input energy having a known frequency.
Many appliances utilize electric motors. For example, a dishwasher has an electric motor which can be used for driving a pump impeller. Also, clothes washing machines and dryers use electric motors.
Objects such as motors have a vibrational output due to the cyclic forces acting therein. An electric motor can vibrate in three directions during operation: axial, radial and rotational, or torsional. Usually, the most significant vibration is torsional.
Torsional vibration of an electric motor is caused by torque pulsations inherent to the operation of AC electric motors. In the United States, AC line voltage has a phase that cycles at 60 Hz. In most other countries, AC line voltage has a phase that cycles at 50 Hz. The torque pulsations within an electric motor are caused by a changing electrical field acting within the motor which occurs at 120 Hz--twice the cycling frequency of the 60 Hz AC power supply (100 Hz for 50 Hz power supply). The changing field causes equal and opposite reaction forces between a rotor and stator of the motor. This causes the rotor to rotate, performing work.
Motor vibration is transmitted through the motor housing to the structure to which the motor can be attached. The transmitted vibration causes structure borne noise. Such noise is undesirable, particularly where the motor is a component of a domestic appliance or in another application intended for a quiet environment. Therefore, it is desirable to reduce the transmission of electric motor vibration to reduce noise.
Known technology for preventing the transmission of motor vibration to a surrounding structure includes resilient isolation mounting, active vibration control systems, reduction of the electromagnetic forces through motor-designed modifications, axis-symmetric internally mounted spring-mass vibration absorbers, and multiple externally mounted axis-parallel spring-mass vibration absorbers.
For instance, U.S. Pat. No. 1,855,570 teaches the use of multiple tuned vibration absorbers mounted to an electric motor housing to reduce vibration. Each vibration absorber comprises a rod and a mass, wherein each rod and mass combination has a natural frequency substantially the same as the vibration which it is intended to reduce. The vibration absorbers are aligned parallel to the axis of rotation.
Also, U.S. Pat. No. 1,834,860 discloses a vibration absorber for an electric motor which has an annular weight mounted internally to the motor housing by multiple springs. The weight and springs are designed to have a natural frequency in the rotational direction corresponding to the rotational vibration of the motor. The weight is aligned on the axis of rotation.
Active vibration cancellation systems are complex and expensive. Other methods, such as with known spring-mass systems, can be less expensive. However, such systems can be inadequate for sufficiently reducing transmitted motor vibration.
A need, however, exists for a simple, inexpensive vibration absorber for effectively reducing vibration of an electric motor and reducing noise transmission to surrounding structures.